Genetics
H S P Support Group Wales 
Genes and chromosomes
Humans have about 50,000 genes. They come in pairs, and each pair makes up a very small section on a chromosomes .
We have 46 chromosomes arranged in 23 pairs in every cell in the body , except for the red blood cells (they lack a cell nucleus ), sperm and egg cells.
Twenty-two of these pairs are called autosome s and they are the same for each sex.
The twenty-third pair of chromosomes consists of the sex chromosomes. Women have two X chromosomes while men have one X and one Y chromosome. The Y chromosome determines maleness. So the man's sperm dictates the sex of the child.
Sperm and egg cells contain just one of the chromosomes from each pair, i.e., 23 individual chromosomes. When a sperm and egg come together and conception begins, a new cell is formed that brings the individual chromosomes together into 23 pairs again. Thus, every person receives half of his or her genes from the father's sperm cell, and half from the mother's the egg cell.
When things go wrong
If the coding to a Gene is faulty it can cause many conditions one of these being HSP. HSP is not a specific condition but a group of conditions that cause spasticity in the legs. In fact more than 30 Genes have been identified as causing this problem and they are all grouped under the umbrella of HSP. The types of mutation and implications are as follows:
AUTOSOMAL DOMINANT. You need only one copy of this Gene to pass on HSP condition.
AUTOSOMAL RECESSIVE. You need two copies of this Gene to pass on HSP condition.
X LINKED INHERITANCE. See below.
Autosomal simply refers to non-sex chromosomes.
Many other factors can influence the outcome of inheritance, but the scenarios below are a good example of the basic understanding of Genetic inheritance.
